undergo π-stacking. Understandably, the rigid structures with
orthogonal planes should face molecular packing problems
such that they crystallize out only in the presence of guest
molecules, which get included in the crystal lattice; this
concept has been exploited by Aoyama and co-workers9 to
develop organic hydrogen-bonded porous materials. A
consequence of this is that such compounds fall out as
amorphous powders in the absence of suitable guest mol-
ecules. The amorphous property is a highly desirable
prerequisite for application of an organic compound as a
functional material in OLEDs.10 The amorphous nature
permits uniform pin-hole-free deposition of thin films in
addition to the fact the indisposed tendency of the molecules
toward crystallization (without guests) obviates creation of
defect sites at which the excitons are nonradiatively quenched;
there is an intimate relationship between the crystallinity of
the film and device performance.11 The fact that the solid-
state fluorescence quantum yields parallel those of the
solution state is suggestive of similar emissive behavior in
both media.
molecular size and rigid non-coplanar aromatic planes
intrinsic to 1-3, which retard crystallization, permit sublima-
tion without any decomposition in the vacuum chamber.
Thus, the vacuum-sublimed devices constructed for 1-3
exhibit bright blue light emission with λmax at ca. 450 nm. It
is noteworthy that the maximum external quantum efficiency
achieved for the non-doped device fabricated for 3 is 3.3%;
this value is comparable to commonly used blue emitting
materials based on spirobifluorene (3.2 cd/A),20a diarylan-
thracenes (2.6-3.0 cd/A),20b,c diphenylvinylbiphenyls (1.78
cd/A),20d biaryls (4.0 cd/A),20e etc. The maximum luminance
efficiency of 2.7 cd/A achieved in the case of 3 underscores
the fact that the attachment of sterically hindered substituents
to the pyrene does indeed lead to suppression of face-to-
face aggregation. It is instructive to note that the emission
is pure blue according to the CIE coordinates. The observed
external quantum yield and luminance efficiencies for 1-3
are respectably high, which warrants further investigations
of analogous derivatives to afford materials with improved
properties.
The 4-fold substitution by 2,6-dimethylaryl rings in 1-3
considerably improves their morphology as well as thermal
stability, as revealed from their thermal decomposition
temperatures (cf. Table 1); for example, Td for 3 is ca. 392
°C. Further, the energy gap between the HOMO and LUMO
of 1-3 is as high as ca. 3.2 eV, which is very advantageous
from the point of view of their application as host materials
for Fo¨rster-type energy transfer in OLEDs; indeed, only a
limited number of wide gap blue emitting materials are
currently available as hosts in light emitting devices.12 Thus,
red, green, and white lights may be conveniently derived
from blue emitting materials with wide band gap energy via
a downhill energy transfer to generate full color displays,
which constitutes an ultimate objective of the contemporary
research on OLEDs.13
In conclusion, we have shown that sterically congested
tetraarylpyrenes 1-3 can be readily accessed via facile
Suzuki coupling and that the 4-fold functionalization mani-
fests in steric inhibition of molecular aggregation in both
solution (UV-vis and fluorescence) and solid states (pho-
toluminescence and crystal packing). The arene units so
attached impart thermal stability and noncrystalline property
to permit the potential of 1-3 as emissive materials in
OLEDs to be readily explored. The devices fabricated for
1-3 lead to pure blue electroluminescence with respectable
device performances.
Acknowledgment. J.N.M. is thankful to DST, India, for
funding. P.N. is grateful to UGC for a research fellowship.
Dr. P. Venugopalan, Chandigarh, is gratefully acknowledged
for the help with structure refinement.
Pyrene-based triarylamines,14 oligoquinolines,15 quinoxa-
lines,16 fluorenes,17 oligothiophenes,18 and octavinylsilses-
quioxane,19 etc. have been reported in the literature as hole-
transporting, emitting, and host materials. The moderate
Supporting Information Available: Experimental pro-
cedures, spectral data (1H and 13C NMR), X-ray structure
determination details, and cif file. This material is available
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